The present invention relates to providing printed circuit boards (PCB's) with a cover coat, and in particular, to a method for creating the cover coat pattern in a photoresist overlayer simultaneously with the pattern for pads, lines, etc. in the copper foil of the board.
In the prior art, PCB's are manufactured in two basic steps. The first step is creating the pattern for the conducting material, e.g. copper, and the second step is to cover the PCB with a protective coating except on the contact pads.
In the first step a polyimide sheet substrate, e.g. of Kapton (Kapton is a registered trademark of E. I. DuPont de Nemours and Company, Wilmington, Del.), is provided with a thin foil or layer of a conductive material, such as copper. The copper is covered with a photoresist (e.g. AZ1375, which is a trademark of Shipley Co., Newton, Mass.) and a photo mask containing the circuit pattern is placed on the resist. The photoresist is exposed to light through the mask, whereby the portions that were exposed to light, are susceptible to removal by a suitable developer, whereas the unexposed areas, defining the circuit pattern, remain. After exposure the mask is removed, and the soluble areas are washed away. The exposed copper is etched away leaving a circuit pattern. The photoresist is again exposed and developed, or removed by a suitable solvent thus finishing the printed circuit panel.
In the second step the normal procedure is to blanket coat a printed circuit panel, e.g. by silk screen printing, with a negative working photo-sensitive material (by negative working is understood that illuminated parts are polymerized, whereas remaining parts can be washed away; an example of such a material is cover coat 5500 which is a trademark of Lea Ronal, Inc., Freeport, N.Y.). A photo mask, with a pattern for those areas to be covered, is aligned to the panel and exposed to light.
This alignment of the mask for the cover coat with those parts of the PCB that are not to be covered, i.e. the pads, is very difficult, especially when there is a high circuit density, and the operation is time consuming and thus very expensive.
Also known in prior art is the use of a special multi-density mask for the pattern generation. Such a mask is disclosed in IBM Technical Disclosure Bulletin Vol. 19, No. 12, May 1977, pages 4539 and 4540, and in Vol. 20, No. 3, August 1979, pages 964-5.
The described mask is such that opaque portion correspond to lands, partially transparent portions correspond to lines and fully transparent portions correspond to background substrate.
The present invention uses a modification of this mask, which will be described in detail later.
In view of the above mentioned disadvantages with prior art techniques, it is the object of the present invention to provide a method for cover coating printed circuit boards wherein the alignment problem is eliminated.